Heretofore, poly methyl methacrylate (hereinafter abbreviated to as PMMA) which is typical acrylic resin is preferably used to an optical film in view of its excellent transparency, dimension stability and low hygroscopic property (Patent Document 1).
However PMMA film has poor heat resistance and has a problem that deformation occurs when used under high temperature or in long-terms usage.
This problem is important not only in a property of film itself but in a property of a polarizing plate and display device. That is, in a liquid display device, a polarizing plate is curled due to a deformation of a film resulting in curving whole panel. Therefore view angle variation or change of color occurs due to change of a retardation, even when panel is used at the position of visible surface.
In order to improve a heat resistance, proposed is a method in which polycarbonate (hereinafter abbreviated to as PC) is added in acrylic resin. However, it is difficult to use as an optical film, because usable solvent is limited and miscibility of resins is not enough and it tends to become clouded (Patent Document 2).
Disclosed is a method for introducing an aliphatic alkyl group as a copolymerization component of an acrylic resin, or a method for forming a cyclic structure in a main molecular chain via intermolecular cyclization reaction (Patent Documents 3 and 4).
However, even though these methods improve a heat resistance, brittleness of a film becomes extremely low and deterioration of brittleness accelerates a deformation of panel. Thus change of a retardation cannot be suppressed and after all, problem about view angle variation or change of color cannot be solved.
Moreover, these problems about transparency, high heat resistance and brittleness becomes more and more pronounced as a recent increase of display size, decrease of thickness of member and reduction in weight.
In the case of laminated film (for examples Patent Documents 5 and 6) of PMMA and a cellulose acetate butylate (hereinafter abbreviated to as CAB), flexibility and dimension stability under high temperature and high humidity can be improved to a certain degree. However when in an actual environment of usage of a display device, for example, a repeated and long-term change of environment between low and high temperature such as between −20° C. and 60° C. results in increasing of haze or occurring of craze at a boundary face of PMMA layer and CAB layer and it is difficult to keep displaying brilliant image stably in long term.    Patent Document 1: Japanese Patent Publication Open to Public Inspection (hereinafter referred to as JP-A) No. 5-119217    Patent Document 2: JP-A No. 5-306344    Patent Document 3: JP-A No. 2005-146084    Patent Document 4: JP-A No. 2007-191706    Patent Document 5: JP-A No. 2007-233114    Patent Document 6: JP-A No. 2007-233215